docs: def A_hollow_cylinder(Di, Do, L):
      r'''Returns the surface area of a hollow cylinder.
  
      .. math::
          A = \pi D_o L + \pi D_i L  + 2\cdot \frac{\pi D_o^2}{4}
          - 2\cdot \frac{\pi D_i^2}{4}
  
      Parameters
      ----------
      Di : float
          Diameter of the hollow in the cylinder, [m]
      Do : float
          Diameter of the exterior of the cylinder, [m]
      L : float
          Length of the cylinder, [m]
  
      Returns
      -------
      A : float
          Surface area [m^2]
  
      Examples
      --------
      >>> A_hollow_cylinder(0.005, 0.01, 0.1)
      0.004830198704894308
      '''
      side_o = pi*Do*L
      side_i = pi*Di*L
      cap_circle = pi*Do**2/4*2
      cap_removed = pi*Di**2/4*2
      return side_o + side_i + cap_circle - cap_removed